I have three meters: the one prescribed by my GP's surgery, one supplied free by a manufacturer whose expensive strips my GP won't prescribe, and my dear old first-ever meter that is a little cracked and unreliable it its firing ability and adds an exciting flavour of Russian roulette to the boring business of BG testing. All three use the same round flat batteries that you can buy in most chemists and supermarkets. I do try to keep a supply of spare batteries, but they're a lot easier to lose than spare meters.
If you're well-organised you're probably fine with just one meter and a spare battery or two. However, it can be helpful to double-check an unexpectedly high or low reading on a different meter.
Kate